260 THE MICROSCOPE. 



them -united in the actively-growing yeast plant ; hut in 

 this case the growth is accelerated, apparently, hy cold and 

 damp. Another plant belonging to the same species, the 

 Protococcus jrtuvialis, is found in every pool of water, the 

 spores of which must be always floating in the air, since 

 it appears after every shower of rain. 



Unicellular plants occur in the series of Fungi and 

 Algae, which have many and very varied correspondence in 

 morphological respects. The unicellular Algae — that is to 

 say, Algse, the contents of which, containing already 

 organized particles, are inclosed in a single, semifluid 

 envelope, and this again in a cell-membrane, often 

 consisting of several layers of different kinds ; and many, 

 moreover, possess the power of dividing into several 

 secondary cells, for the most part equivalent to the primary 

 cell. To this species of unicellular plant belongs Protococcus 

 pluvialis. That this is the case is clearly seen in the still 

 form of this plant, which is most distinctly characterised 

 by its cell-membrane, a more or less thick though always 

 colourless envelope. It never, however, secretes true 

 thickening layers on the surface. Although this cell- 

 membrane exhibits all the optical characters of one com- 

 posed of cellulose, it is impossible to demonstrate the 

 presence of that principle by means of iodine and sulphuric 

 acid ; it is not coloured by those reagents even after the 

 contents of the cell have been expressed. 



The contents vary much in consistence, colour, solid 

 and fluid constituents ; the red and green portions of 

 which appear to be of equal physiological importance. 

 The green colour is removed by ether, on the evaporation 

 of which solvent there remain green as well as colourless 

 drops. Dilute sulphuric acid at first renders the colour 

 paler; but its prolonged action produces a bright green 

 hue, which gradually becomes more and more intense, and 

 often almost a blue-greem Hydrochloric acid has a simi- 

 lar effect ; a tinge of brown is produced by nitric acid. 

 Carbonate of potash scarcely affects the green colour ; it 

 is gradually but totally destroyed by caustic potash, the 

 contents at the same time swelling and becoming 

 transparent. 



The change of colour from green to red in Euglena 



